Aluminum-silicon (Al-Si) based alloys are widely used as foundry alloys for a variety of different applications. For example, engine blocks and pistons for air compressors employed in the automotive industry are cast from Al-Si based alloys. Casting alloys are distinguished from wrought alloys which contain 95% or more aluminum and are not used for castings but are used for applications such as can stock, gutters, siding, airplane skins, etc. Casting alloys can be broken down into two general categories:
(a) Hypoeutectic alloys which contain silicon in weight percent amounts of up to but less then 12 percent; and
(b) Hypereutectic alloys which contain more than 12 percent by weight of silicon.
Typical hypoeutectic foundry alloys are alloys identified in the trade by the nomenclature A319 and A356 alloys, which contain approximately 7 percent by weight of silicon. In a similar fashion, a typical hypereutectic foundry alloy is identified by the nomenclature A390 alloy which contains approximately 17 percent by weight of silicon. In addition to alleviating the formation of defects such as shrinkage of the alloy, and porosity due to the gases and also to minimize the presence of inclusions, two very significant ways in which the strength and performance of an aluminum casting alloy can be improved is through grain refinement of the alloy and modification of the eutectic structure.
Grain refinement is simply the process of adding nuclei to the melt prior to pouring, such that upon the freezing process (i.e., the solidification journey) the casting will expedite a fine grained microstructure. Grain refinement is accomplished by adding master alloys containing titanium (Ti) and/or boron (B).
Eutectic modification, on the other hand, is the process of changing the morphology of the cast structure; moreover, that portion of the cast alloy which freezes last. In other words, the last liquid to freeze which is of eutectic composition, if unmodified, will result in a cast structure containing large acicular and deleterious Si-rich crystals. The morphology of these Si crystals is modified (hence the term modification) by the addition of sodium, or strontium, or antimony to the melt, to alter the eutectic structure and to yield fine Si-rich crystals having a fibrous structure.